Okay let me answer my own question. The problem is that my parent-child
relationship does not have the delete-orphan cascade. So when I set the new
children, the old child_2 loses its parent (as is expected, because it's no
longer a child) and then there's an error because the DB has a not null
constraint on the parent_pk coming from nullable=False.
I guess the moral of the story is that parent_pk being non-nullable
essentially requires delete-orphan.
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